1. Field of the Invention
The invention relates to image intensifier tubes, notably of the radiological type. It relates particularly to optical means which can be used to correct the distribution of the luminous intensity at output of the image intensifier tube.
2. Description of the Prior Art
Image intensifier tubes are vacuum tubes comprising an input screen, located at the front of the tube, an electronic optical system and a screen for the observation of the visible image located at the rear of the tube, on the same side as an output window of this tube.
In radiological image intensifier tubes, the input screen further has a scintillator screen which converts the incident X photons into visible photons.
The visible photons excite a photocathode which generates a flux of electrons in response. This flux of electrons is then transmitted by the optical electronic system which focuses the electrons and directs them to the observation screen, more specifically to a cathodoluminescent screen generally constituted by one or more layers of grains of phosphor or luminophore. The cathodoluminescent screen then emits a visible light.
FIG. 1 gives a schematic view of a radiological type of image intensifier tube such as this.
The intensifier tube 1 comprises a glass envelope 2, one end of which, at the front of the tube, is closed by an input window 3 exposed to a radiation of X photons.
The second end of the envelope forming the rear of the tube is closed by an output window 4 transparent to light.
The X rays are converted into light rays by a scintillator screen 5. The light rays excite a photocathode 6 which produces electrons in response. These electrons are extracted from the photocathode 5 and are accelerated towards the output window 4 by means of different electrodes 7, and by an anode 8 positioned along a longitudinal axis 9 of the tube and forming the electronic optical system.
In the example shown, the output window 4 is formed by a transparent glass element attached to the envelope 2 in an imperviously sealed way. In the example shown, this glass element furthermore constitutes a support which bears a cathodoluminescent screen 10, made of luminophores for example.
The impact of the electrons on the cathodoluminescent screen 10 makes it possible to reconstitute an image (amplified in luminance) which, at the outset, was formed on the surface of the photocathode 6. The output window 4, made of glass, forms a part of the envelope 2, in such a way that an external face 13 of this window, opposite the luminophore layer 10, constitutes an external part of the tube 1.
The image displayed on the luminophore layer 10 is visible through the glass element which constitutes the output window 4. Generally, optical sensor devices (not shown) are positioned outside the tube in the vicinity of the output window 4 to pick up this image through the window 4 and enable it to be observed.
The intensifier tube 1 may further comprise, as in the example shown, a transparent strip or plate 14 fixedly joined by a layer of bonder 15 to the external face 13 of the output window 4. The transparent plate 14 has the function of improving the contrast and, to this end, it has a non-negligible thickness E of 10 mm for example. The result thereof is that light rays 16, emitted by the luminophore layer 10, forming substantial angles with a ray normal to the plane of the transparent plate 14, tend to emerge from this plate towards its edges, and thus tend to go out of the field of the optical sensor device (not shown) mentioned here above.
For reasons related notably to electronic optics, the surface of the input screen, namely the surfaces of the input screen 3 as well as of the scintillator and of the photocathode 5, 6 are not plane but bulging. The result thereof is that if the input window 3 is illuminated by a uniform beam of X-rays, the electronic density generated by the input screen is not uniform and this has repercussions, at the output of the tube, on the brightness curve along a diameter D at the output window 4: the brightness curve represents the luminous intensity at each point of the diameter D of the output window 4.
It is observed that this curve is generally shaped like the arc of a circle: the brightness is the maximum towards the center and diminishes appreciably as the edges are approached. The diminishing of brightness at the edges is commonly between 25% and 30% and may attain 35% for intensifier tubes having large-sized input windows.
In the case of radiological image intensifier tubes, it has already been proposed in the prior art (in the European patent document EP 0 239 991) to improve the homogeneity of the brightness by giving a non-homogeneous distribution to the thickness of the layer that constitutes the scintillator 5. This method gives fairly good results, but its implementation on an industrial scale is a delicate and unwieldly affair, notably because the efficiency of a scintillator varies with its thickness.